The Garmin Edge 305 is a GPS device for cycles that is able to provide speed, distance, direction and altitude information. Options are available for heart rate and pedalling speed also. There is an extremely accurate instantaneous readout and the journey statistics can be downloaded on to a computer (which will show the route on a crude map). It is not a SatNav system, although it is extremely good as a training tool. It is quite small and unobtrusive on the frame or handlebars.
It is quite expensive however and you can buy a reasonable bike for around the same price. The Garmin Edge 205 is cheaper, although it does not have the heart rate/pedalling speed options and the altitude measurement is less accurate (relying on GPS rather than barometric pressure).
Having bought a Garmin Edge 305, I wonder how I ever got by without one.